LNG production is a rapidly growing means to supply natural gas from locations with an abundant supply of natural gas to distant locations with a strong demand of natural gas. The conventional LNG cycle includes: (a) initial treatments of the natural gas resource to remove contaminants such as water, sulfur compounds and carbon dioxide; (b) the separation of some heavier hydrocarbon gases, such as propane, butane, pentane, etc. by a variety of possible methods including self-refrigeration, external refrigeration, lean oil, etc.; (c) refrigeration of the natural gas substantially by external refrigeration to form LNG at near atmospheric pressure and about −160° C.; (d) transport of the LNG product in ships or tankers designed for this purpose to a market location; and (e) re-pressurization and re-gasification of the LNG to a pressurized natural gas that may distributed to natural gas consumers. Step (c) of the conventional LNG cycle usually requires the use of large refrigeration compressors often powered by large gas turbine drivers that emit substantial carbon and other emissions. Large capital investments—on the order of billions of US dollars—and extensive infrastructure may be required as part of the liquefaction plant. Step (e) of the conventional LNG cycle generally includes re-pressurizing the LNG to the required pressure using cryogenic pumps and then re-gasifying the LNG to form pressurized natural gas by exchanging heat through an intermediate fluid but ultimately with seawater, or by combusting a portion of the natural gas to heat and vaporize the LNG. Generally, the available exergy of the cryogenic LNG is not utilized.
A cold refrigerant produced at a different location, such as liquefied nitrogen gas (“LIN”), can be used to liquefy natural gas. A process known as the LNG-LIN concept relates to a non-conventional LNG cycle in which at least Step (c) above is replaced by a natural gas liquefaction process that substantially uses liquid nitrogen (LIN) as an open loop source of refrigeration and in which Step (e) above is modified to utilize the exergy of the cryogenic LNG to facilitate the liquefaction of nitrogen gas to form LIN that may then be transported to the resource location and used as a source of refrigeration for the production of LNG. U.S. Pat. No. 3,400,547 describes shipping liquid nitrogen or liquid air from a market place to a field site where it is used to liquefy natural gas. U.S. Pat. No. 3,878,689 describes a process to use LIN as the source of refrigeration to produce LNG. U.S. Pat. No. 5,139,547 describes the use of LNG as a refrigerant to produce LIN.
The LNG-LIN concept further includes the transport of LNG in a ship or tanker from the resource location to the market location and the reverse transport of LIN from the market location to the resource location. The use of the same ship or tanker, and perhaps the use of common onshore tankage, are expected to minimize costs and required infrastructure. As a result, some contamination of the LNG with LIN and some contamination of the LIN with LNG may be expected. Contamination of the LNG with LIN is likely not to be a major concern as natural gas specifications (such as those promulgated by the United States Federal Energy Regulatory Commission) for pipelines and similar distribution means allow for some inert gas to be present. However, since the LIN at the resource location will ultimately be vented to the atmosphere, contamination of the LIN with LNG (which, when regasified as natural gas, is a greenhouse gas more than 20 times as impactful as carbon dioxide) must be reduced to levels acceptable for such venting. Techniques to remove the residual contents of tanks are well known but it may not be economically or environmentally acceptable to achieve the needed low level of contamination to avoid treatment of the LIN or vaporized nitrogen at the resource location prior to venting the gaseous nitrogen (GAN). What is needed is a method of using LIN as a coolant to produce LNG, where if the LIN and the LNG use common storage facilities, any natural gas remaining in the storage facilities is effectively purged prior to filling the storage facilities with LIN.